This invention relates to temperature sensors, and more particularly to a substantially solid optical temperature transducer which absorbs light as a function of its temperature. Temperature transducers of this general type are known in the prior art, and some configurations are shown in which fiber optical light guides provide for locating the transducer remotely from the detector and read-out portions of the temperature detecting system.
U.S. Pat. No. 2,824,235 (Hahn) shows using the temperature-dependent light absorbing property of a semiconductor for measuring radiant energy. U.S. Pat. No. 3,672,221 (Weil), by using the variation in the index of refraction of a semiconductor with temperature, shows how such a semiconductor may be used to measure temperature with long-wave polarized light for which the semiconductor has little absorption. Techniques such as these, however, require precision spatial orientation of the various system components.
U.S. Pat. No. 3,750,155 (Keller et al) discloses a colorimeter probe which uses fiber optics and a cone or pyramid to direct a light beam from an emitter to a light-modulating means, redirect it 180 degrees, and return it to a detector. U.S. Pat. No. 3,960,017 (Romanowski), also using fiber optics, interrupts a light beam passing from one fiber to another by using differential thermal expansions to cause a sensing rod to block the light crossing a gap in the fiber optics, as a function of temperature. This requires the emitting and detecting fibers to have their ends curved and positioned to face one another, or the use of beam directing prisms.
U.S. Pat. No. 4,016,761 (Rozzell et al) provides a structure in which the emitting and detecting fibers of the fiber optical bundle can remain parallel, by making use of the temperature dependent reflectance of liquid crystals to return the light from the emitting to the detecting fibers. U.S. Pat. No. 4,036,606 (Deficis) also uses parallel optical fibers and a liquid detector, here using the reflectance of a variable liquid meniscus to indicate temperature. U.S. Pat. No. 4,075,493 (Wickersheim) discloses a temperature probe in which a mixture of phosphors is affixed to the end of parallel fibers.
Another example of a semiconductor transducer using parallel fibers is provided by U.S. Pat. No. 4,140,393 (Cetas) in which the refractive properties of a birefringent crystal at the end of the probe are utilized by placing a polarizer and a mirror at the transducer end. Another example is in U.S. Pat. No. 4,136,566 (Christensen) in which a semiconductor transducer shaped like a prism is attached to the ends of parallel fibers to redirect light from the emitting fibers into the detecting fibers.
As will be appreciated, these prior art devices are generally fairly complicated, and require particular care in construction and assembly. Hahn and Weil need precisely aligned components to direct a beam of light to and from the thermally responsive element. Romanowski needs precisely bent and aligned fibers, or precisely aligned prisms. Rozzell, Deficis, Cetas, Christensen and Wickersheim avoid these problems by placing the thermally responsive transducer directly at the end of a fiber optic bundle. The liquid crystal transducer of Rozzell obviates the need for a particular mechanical orientation because the liquid crystal does not require such a specific orientation to the fibers. However, care in assembly of the Rozzell device is required to keep the transducer cavity optimal, an airtight chamber with optically smooth surfaces is required, and the liquid crystal transducer may suffer from instability due to exposure to wide temperature excursions.
Deficis, while being free from the need for specific transducer-to-fiber orientation, nevertheless requires a precision chamber and complete chamber isolation from the surrounding environment. There may also be some problems associated with stability of the liquid used in the transducer. Cetas gains the inherent stability of the birefringent crystal properties, but the polarizer properties may not be as predictable, and the transducer requires careful crystal preparation and crystal-to-polarizer orientation, as well as a mirror.
Christensen has avoided the problems of liquids, precision cavities, mirrors, and polarized light, through the use of a solid prism. However, the preparation of the prism itself requires precision, and the optical coupling of the prism to the fibers requires care and surface preparation. It may also present difficulties in maintaining coupling stability because of the relatively large planar coupling area where differential contraction and expansion may occur.
Wickersheim has achieved even greater ease of fabrication by simply affixing a phosphor mixture to an end of a bundle without regard to orientation, and his ratiometric technique frees this approach from concern with coupling stability. The limits on the phosphor emittance and the need for careful filtering of the two frequencies may require complications in detection and circuitry.
What is needed, therefore, is an optical temperature transducer which is compatible with parallel optical fibers, which has the same freedom from coupling problems and the same ease of assembly of the liquid crystal and phosphor transducers, and which at the same time provides the detection stability and simplicity of the solid semiconductor transducers. Such a device should also be inexpensive and readily suited for convenient, uncomplicated mass production.